Generally, a capacitor is a general electric part and has widely been used in power circuits and noise filters for digital circuits in various electric and electronic devices. Various electrolytic capacitors have been used and examples include an aluminum electrolytic capacitor, a wet tantalum electrolytic capacitor and the like. An aluminum electrolytic capacitor is expected to provide a particularly excellent function using the present invention. Therefore, the present invention will now be described with reference to this kind of electrolytic capacitor.
A conventional aluminum electrolytic capacitor can be produced by using an anode foil which is made by etching a high-purity aluminum foil, to increase its surface area, and anodizing the surface of the aluminum foil to provide a film, and a cathode foil whose surface has only been etched. The resulting anode foil and cathode foil are disposed opposite to each other and an element with a wound structure is made by interposing a separator (release paper) between the foils, followed by winding, and then an element made by winding the element with a wound structure is impregnated with an electrolyte solution. The element impregnated with the electrolyte solution is contained in a case (generally made of aluminum), which is then sealed with an elastic sealant, thus completing an electrolytic capacitor. An electrolytic capacitor may have a structure other than a wound structure.
In the above-described electrolytic capacitor, the characteristics of the electrolyte solution may be factors which decides the performances of the electrolytic capacitor. A conventional electrolyte solution for use in an electrolytic capacitor is generally prepared by dissolving, as an electrolyte, a carboxylic acid such as adipic acid, benzoic acid, etc. or an ammonium salt thereof into a solvent prepared by adding about 30% by weight or less of water to ethylene glycol (EG) as a principal solvent. Such an electrolyte solution has a specific resistance of about 1.0 Ω·m (100 Ω·cm). Such a specific resistance is insufficient for recent small, high-performance electrolytic capacitors. Thus, a highly conductive electrolytic capacitor having a low specific resistance is required.
Intensive studies have been made in this field to improve the characteristics of the electrolytic solution, and various electrolytic solutions for use in an electrolytic capacitor have already been proposed. Referring to the patent documents which have recently been laid open to public inspection, there have been proposed the following aluminum electrolytic solutions for use in an electrolytic capacitor.
An aluminum electrolytic solution for use in an electrolytic capacitor, which contains 20 to 90% by weight of water, comprising at least one ammonium salt compound selected from the group consisting of ammonium salt compounds such as ammonium formate, as a principal electrolyte, and 1% by weight or more of at least one carboxylic acid compound selected from the group consisting of carboxylic acid compounds such as trimethyladipic acid which has no fire point, wherein a solidification point is −10° C. or lower (see, Japanese Unexamined Patent Publication (Kokai) No. 2000-188240).
An aluminum electrolytic solution for use in an electrolytic capacitor, comprising a solvent containing 35 to 100% by weight of water and a chelating agent (see, Japanese Unexamined Patent Publication (Kokai) No. 2000-331886).
An aluminum electrolytic solution for use in an electrolytic capacitor, comprising a solvent containing 35 to 100% by weight of water and a bonded product formed by bonding phosphoric acid ions to a water-soluble complex comprising aminopolycarboxylic acid and ammonium (see, Japanese Unexamined Patent Publication (Kokai) No. 2001-319833).
Aluminum electrolytic solution for use in an electrolytic capacitor, comprising a solvent containing 35 to 100% by weight of water, at least one of adipic acid and a salt thereof dissolved in the solvent, and a bonded product formed by bonding phosphoric acid ions to a water-soluble complex comprising tannin and/or a decomposition product of tannin and aluminum (see, Japanese Unexamined Patent Publication (Kokai) No. 2002-83743).
An aluminum electrolytic solution for use in an electrolytic capacitor, comprising a solvent containing 35 to 100% by weight of water and a water-soluble aluminum complex bonded with phosphoric acid ions (see, International Publication No. WO00/55876).
It can be seen that these electrolytic solutions for use in an electrolytic capacitor and other electrolytic solutions have improved working life or durability at high temperature and other characteristics. However, some room for improvement is left. For example, a conventional electrolytic solution is inferior in noise removing ability and in charging/discharging efficiency because the equivalent series resistance (ESR) of a capacitor cannot be decreased sufficiently. Therefore, the resulting electrolytic capacitor cannot be used advantageously as an electronic part. Also an electrolytic capacitor using a conventional electrolytic solution has a problem that the electrical characteristics are unstable over a wide temperature range from a low temperature just above the freezing temperature to a high temperature and also the ESR drastically varies. Furthermore, a conventional electrolytic capacitor has a problem that swelling and liquid leakage from the case containing the capacitor element sometimes occurs and the working life at a high temperature is short.